"Morse code" PCM transmissions from Soviet reconnaissance satellites

Starting with Kosmos 251 a new type of transmission appeared consisting of 12 groups, each of 3 Morse code characters. This signal was normally transmitted on 19.150 MHz, but sometimes appeared on 19.300 MHz (when there were two similar satellites in orbit ant the same time) and once on 19.995 MHz. Each group was formed of characters giving seven pulses per group in a 2-3-2 configuration with the exception of one 3-1-3 group which served to denote the start of the frame. These represented 7-bit pulse code modulation binary code with the most significant bit transmitted first. The sixth group , which was always MWI during the main part of the mission, changed to MRI on the day prior to recovery at the same time an additional piece was catalogued in orbit. The numerical value of the seventh group increased steadily during the flight and was shown to be a film exposure counter. Click here to listen to the signal (68 kB mp3-file).

Source: chapter 5 "Soviet Military Space Activities" in "Soviet Space Programs: 1976-80, Part 3, US Govt. Printing Office, May 1985.

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